None.
This invention relates to the fabrication of three-dimensional objects using extrusion-based layered manufacturing techniques. More particularly, the invention relates to supplying solid modeling material to a liquifier carried by an extrusion head, and extruding the material in a flowable state in a predetermined pattern in three dimensions with respect to a base.
Three-dimensional models are used for functions including aesthetic judgments, proofing a mathematical model, forming hard tooling, studying interference and space allocation, and testing functionality. Extrusion-based layered manufacturing machines build up three-dimensional models by extruding solidifiable modeling material from an extrusion head in a predetermined pattern, based upon design data provided from a computer aided design (CAD) system. Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Danforth et al. U.S. Pat. No. 5,738,817, Batchelder et al. U.S. Pat. No. 5,764,521 and Dahlin et al. U.S. Pat. No. 6,022,207, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
A feedstock of either a liquid or solid modeling material is provided to the extrusion head. Where the feedstock of modeling material is in solid form, the extrusion head brings the feedstock to a flowable temperature for deposition. One technique provides the modeling material to the extrusion head in the form of a filament strand.
In the Stratasys FDM(copyright) modeling machines of the current art which employ a filament feed, modeling material is loaded into the machine as a flexible filament wound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. A solidifiable material which adheres to the previous layer with an adequate bond upon solidification and which can be supplied as a flexible filament is used as the modeling material. Motor-driven feed rollers advance the strand of the filament into a liquifier carried by an extrusion head. Inside the liquifier, the filament is heated to a flowable temperature. Flowable modeling material is forced out of a nozzle on the far end of the liquifier, and deposited from the liquifier onto a base. The motor-driven feed rollers pushing filament into the liquifier create a xe2x80x9cliquifier pumpxe2x80x9d, wherein the filament itself serves as the piston. As the feed rollers advance filament into the liquifier, the force of the incoming filament strand extrudes the flowable material out from the nozzle. The flow rate of the material extruded from the nozzle is a function of the rate at which the filament is advanced to the head. The flow rate is commanded by controlling the speed of advancement of filament into the liquifier. A controller controls movement of the extrusion head in a horizontal x, y plane, controls movement of the base in a vertical z-direction, and controls the rate at which the feed rollers advance filament. By controlling these processing variables in synchrony, the modeling material is deposited in xe2x80x9cbeadsxe2x80x9d layer-by-layer along tool paths defined from the CAD model. The material being extruded fuses to previously deposited material and solidifies to form a three-dimensional object resembling the CAD model.
The extruded material delivered by the liquifier pump has a bead of a cross-sectional area that should ideally be controlled to create an accurate model. Usually, a constant bead width is desired. The bead width is related to the flow rate of material out of the pump as well as the extrusion head velocity. The bead width is also affected by the clearance between the extruding nozzle tip and a previously extruded layer (or the base). If the head velocity were to change while the flow rate were to stay constant, the bead width would vary as well.
One type of rapid prototyping system of the prior art drives the motion of the extrusion head at a constant velocity along a tool path comprising a poly-line. A poly-line is a continuous curve of straight-line segments defined by a list of X-Y coordinate pairs at each vertex. The head velocity is preselected so as to accomplish the general goal of moving the extrusion head quickly along the poly-line while minimizing the displacement from the tool path. As a result, the head velocity must be set to be slow enough that the deviation will not exceed the maximum allowable following error for the largest deflection along that poly-line. Using a constant head velocity along a tool path, bead width remains fairly constant but errors arise at start points and end points of the tool path, for instance, at the location of a xe2x80x9cseamxe2x80x9d (i.e., the start and end point of a closed-loop tool path).
Another type of prototyping system of the prior art varies the extrusion head speed to increase the throughput of the modeling machine. The extrusion head speeds up along straight-aways in the tool path, and slows down where there are deflection angles or vertices. U.S. Pat. No. 6,054,077 describes one such technique for varying the extrusion head speed, using X-Y trajectory profiling that follows the exponential step response of the liquifier pump. The velocity profile of the extrusion head looks like a xe2x80x9cshark toothxe2x80x9d, while the pump profile follows a step function.
It has been observed that the variable velocity systems of the prior art introduce greater bead width error, and also have seam errors. It would be desirable to reduce errors in bead width and seam quality so as to achieve a desired extrusion profile, while allowing the higher throughput of a variable rate system.
The present invention is a liquifier pump control method and apparatus which reduces bead width errors and seams errors observed in the prior art by accounting for thermal expansion of the modeling material in the liquifier. The melting of modeling material is accompanied by its expansion. The present invention recognizes that the melt expansion produces unanticipated extruded flow rates from the liquifier during transient conditions. The present invention predicts a melt flow component of the extruded flow rate produced by the thermal expansion of the modeling material, and compensates for the predicted melt flow in a commanded flow rate.